As a conventional technology related to an optical amplifier, technologies described in Japanese Laid-open Patent Publication No. 2009-81473 and Japanese Laid-open Patent Publication No. 05-90671 are known.
In these Patent Documents, output light (excitation light) of one excitation light source is divided by an optical branching device, and the branched excitation light is entered into each of erbium-doped optical fibers (EDF) of a two-stage configuration.
A branching ratio of excitation light by an optical branching device may be variable. For example, Japanese Laid-open Patent Publication No. 2007-127988 describes a branch switching type optical splitter having a variable branching ratio of input light.
In the afore-mentioned technology, actively controlling (or adjusting) a branching ratio of excitation light is under study.